Discontinuous reception (DRX) and discontinuous transmission (DTX) were introduced in CELL_DCH state as part of the continuous packet connectivity (CPC) feature of third generation partnership project (3GPP) Release 7. DRX and DTX, applied to wireless transmit/receive units (WTRUs) in CELL_DCH state, allow the radio access network to maintain temporarily inactive WTRUs in CELL_DCH state to reduce the interference caused by the WTRUs, (i.e., reduce the wasted system capacity), and reduce the WTRU battery consumption. In Release 7, a two-tiered DRX scheme was introduced in CELL_PCH state. Upon transition to CELL_PCH a WTRU enters a DRX mode using a shorter DRX cycle and after a period of inactivity the WTRU switches to a longer DRX cycle.
As part of the development of 3GPP Release 8, it has been agreed to enhance the WTRU operation in CELL_FACH state by allowing enhanced dedicated channel (E-DCH) transmission in lieu of random access channel (RACH), and by enabling DRX operation. WTRUs would use the legacy RACH ramp-up procedure to request a resource assignment, and would receive an acknowledgement indicating the E-DCH resource that has been allocated for the WTRUs. The resource is made up of a set of uplink channels (E-DCH dedicated physical data channel (E-DPDCH), E-DCH dedicated physical control channel (E-DPCCH)) and downlink channels (E-DCH HARQ indicator channel (E-HICH), E-DCH relative grant channel (E-RGCH), E-DCH absolute grant channel (E-AGCH)). The resource is taken from a small pool of shared E-DCH resources managed by the Node B. Once the WTRU completes uplink transmission, the resource is returned to the pool to be assigned to another WTRU. In addition, it was introduced to define a fixed DRX pattern that is in constant use for the whole time a WTRU is in CELL_FACH state except when a WTRU has E-DCH resources allocated. The DRX pattern comprises receive bursts followed by DRX periods. During these DRX periods, the network guarantees that it will not schedule high speed downlink shared channel (HS-DSCH) traffic to this WTRU.
A WTRU in CELL_FACH is required to perform measurements on cells on other frequencies and other radio access technologies (RATs). Such measurements are referred to as inter-frequency and inter-RAT measurements. The criteria to start these measurements and the cells to monitor are specified through broadcast system information. In order to allow the WTRU to measure cells on other frequencies and other RATs, the WTRU is provided with measurement gaps in the downlink transmissions. During these measurement gaps, the WTRU may tune its receiver to other frequencies or other RATs and make the necessary measurements.
While in CELL_FACH, the measurement gaps are referred to as FACH measurement occasions. The FACH measurement occasions define radio frames during which the network does not schedule any data for a WTRU, allowing the WTRU to take measurements on other frequencies or RATs.
When HS-DSCH reception in CELL_FACH is not supported, the FACH measurement occasions are defined to occur during frames where the SFN satisfies the following:SFN div N=C_RNTI mod M_REP+n*M_REP;  Equation (1)where C_RNTI is a cell specific WTRU address (cell RNTI), M_REP is the measurement occasion cycle length=2k, k is the coefficient broadcast in the system information, N denotes the number of 10 ms frames in the measurement occasion, and n=0, 1, 2, 3 . . . .
When HS-DSCH reception in CELL_FACH is supported, the FACH measurement occasions are defined to occur during frames where the SFN satisfies the following:SFN=H-RNTI mod M_REP+n*M_REP;  Equation (2)where H-RNTI is an HS-DSCH radio network temporary identity which is the value stored in the variable H_RNTI, M_REP is the measurement occasion cycle length=2k, and k is the FACH measurement occasion cycle length coefficient. When HS-DSCH reception in CELL_FACH is supported, a FACH measurement occasion of 10 ms-frame: is repeated every M_REP frame.
While in CELL_FACH, the network may be required to send common traffic to all WTRUs. This common traffic may be either control-plane data or user-plane data. The control-plane data may be for all control traffic over broadcast control channel (BCCH)/common control channel (CCCH). The control-plane data includes the following radio resource control (RRC) messages: CELL UPDATE CONFIRM, URA UPDATE CONFIRM, SYSTEM INFORMATION CHANGE INDICATION, or the like.
In addition, a number of multimedia broadcast multicast services (MBMS) related control messages over MBMS control channel (MCCH) and MBMS scheduling channel (MSCH) may require transmission to all WTRUs in CELL_FACH including: MBMS ACCESS INFORMATION, MBMS Common P-T-M RB Information, MBMS Current Cell P-T-M RB Information, MBMS General Information, MBMS Modified services Information, MBMS Neighbouring Cell P-T-M RB Information, MBMS Scheduling Information, MBMS Unmodified services Information, or the like. As for the user-plane data, this refers to traffic over common traffic channel (CTCH) and MBMS traffic channel (MTCH) for cell broadcasting services (CBS) and MBMS type services.
As the WTRU can keep the allocated E-DCH resources for some time, it is possible that this time overlaps with one of the WTRU FACH measurement occasions. If the WTRU is allowed to use the occasion for inter-frequency and inter RAT measurements, then the WTRU will not have access to any of the downlink signaling channels (E-HICH, E-RGCH, E-AGCH) associated with the E-DCH resource. This is further complicated if the WTRU is operating with DRX enabled. The measurement occasions may fall on a DRX period, which may result in a failure of the WTRU taking inter-frequency or inter-RAT measurements because the WTRU receiver is not turned on. If the measurement occasions do not fall on the DRX period, this may result in the WTRU spending prolonged period of time not measuring the downlink channels. More specifically, if the WTRU just performs a DRX and then has to take a measurement on the other frequency the WTRU may not be able to receive any downlink traffic for DRX period plus measurement period. This may limit the data rates on the network side and cause additional delays.
Another problem occurs with common traffic that is to be sent to CELL_FACH WTRUs. As the DRX patterns are tied to the WTRU identity, the DRX patterns will not be synchronized between the WTRUs. As a result, there is no way for the network to transmit common traffic to all WTRUs within their awake times, (i.e., receive bursts). For BCCH traffic, (e.g., the SYSTEM INFORMATION CHANGE INDICATION message), the network may simply repeat the message to all WTRUs, (i.e., sending the message in enough continuous reception (CRX) periods in order to guarantee that all WTRUs have received it). However, this leads to a waste of downlink capacity as the message has to be repeated. Furthermore, if the WTRU DRX patterns are subsets of each other, (e.g., if the pattern of one WTRU repeats twice in the pattern of another WTRU), a WTRU may receive multiple BCCH messages.